In the past there have been three techniques for optically inspecting wafers. Generally they are brightfield illumination, darkfield illumination and spatial filtering.
Broadband brightfield is a proven technology for inspecting pattern defects on a wafer with the broadband light source minimizing contrast variations and coherent noise that is present in narrow band brightfield systems. The most successful example of such a brightfield wafer inspection system is the KLA Model 2130 (KLA Instruments Corporation) that can perform in either a die-to-die comparison mode or a repeating cell-to-cell comparison mode. Brightfield wafer inspection systems, however, are not very sensitive to small particles.
Under brightfield imaging, small particles scatter light away from the collecting aperture, resulting in a reduction of the returned energy. When the particle is small compared to the optical point spread function of the lens and small compared to the digitizing pixel, the brightfield energy from the immediate areas surrounding the particle usually contribute a lot of energy, thus the very small reduction in returned energy due to the particle size makes the particle difficult to detect. Further, the small reduction in energy from the small particle is often masked out by reflectivity variations of the bright surrounding background such that small particles cannot be detected without a lot of false detections. Also, if the small particle is on an area of very low reflectivity, which occurs for some process layers on wafers and always for reticles, photomasks and flat panel displays, the background return is already low thus a further reduction due to the presence of a particle is very difficult to detect.
Many instruments currently available for detecting small particles on wafers, reticles, photo masks, flat panels and other specimens use darkfield imaging. Under darkfield imaging, flat, specular areas scatter very little signal back at the detector, resulting in a dark image, hence the term darkfield. Meanwhile, any presence of surface features and objects that protrude above the surface scatter more light back to the detector. In darkfield imaging, the image is normally dark except areas where particles, or circuit features exist.
A darkfield particle detection system can be built based on the simple assumption that particles scatter more light than circuit features. While this works well for blank and unpatterned specimens, in the presence of circuit features it can only detect large particles which protrude above the circuit features. The resulting detection sensitivity is not satisfactory for advanced VLSI circuit production.
There are instruments that address some aspects of the problems associated with darkfield. One instrument, by Hitachi, uses the polarization characteristics of the scattered light to distinguish between particles and normal circuit features. This is based on the assumption that particles depolarize the light more than circuit features during the scattering process. However, when the circuit features become small, on the order of, or smaller than, the wavelength of light, the circuit can depolarize the scattered light as much as particles. As a result, only larger particles can be detected without false detection of small circuit features.
Another enhancement to darkfield, which is used by Hitachi, Orbot and others, positions the incoming darkfield illuminators such that the scattered light from circuit lines oriented at 0.degree., 45.degree. and 90.degree. are minimized. While this works on circuit lines, the scattering light from corners are still quite strong. Additionally, the detection sensitivity for areas with dense circuit patterns has to be reduced to avoid the false detection of corners.
Another method in use today to enhance the detection of particles is spatial filtering. Under plane wave illumination, the intensity distribution at the back focal plane of a lens is proportional to the Fourier transform of the object. Further, for a repeating pattern, the Fourier transform consists of an array of light dots. By placing a filter in the back focal plane of the lens which blocks out the repeating light dots, the repeating circuit pattern can be filtered out and leave only non-repeating signals from particles and other defects. Spatial filtering is the main technology employed in wafer inspection machines from Insystems, Mitsubishi and OSI.
The major limitation of spatial filtering based instruments is that they can only inspect areas with repeating patterns or blank areas. That is a fundamental limitation of that technology.
In the Hitachi Model IS-2300 darkfield spatial filtering is combined with die-to-die image subtraction for wafer inspection. Using this technique, non-repeating pattern areas on a wafer can be inspected by the die-to-die comparison. However, even with die-to-die comparison, it is still necessary to use spatial filtering to obtain good sensitivity in the repeating array areas. In the dense memory cell areas of an wafer, the darkfield signal from the circuit pattern is usually so much stronger than that from the circuit lines in the peripheral areas that the dynamic range of the sensors are exceeded. As a result, either small particles in the array areas cannot be seen due to saturation, or small particles in the peripheral areas cannot be detected due to insufficient signal strength. Spatial filtering equalizes the darkfield signal so that small particles can be detected in dense or sparse areas at the same time.
There are two major disadvantages to the Hitachi darkfield/spatial filtering/die-to-die inspection machine. First, the machine detects only particle defects, no pattern defects can be detected. Second, since the filtered images are usually dark without circuit features, it is not possible to do an accurate die-to-die image alignment, which is necessary for achieving good cancellation in a subtraction algorithm. Hitachi's solution is to use an expensive mechanical stage of very high precision, but even with such a stage, due to the pattern placement variations on the wafer and residual errors of the stage, the achievable sensitivity is limited roughly to particles that are 0.5 .mu.m and larger. This limit comes from the alignment errors in die-to-die image subtraction.
Other than the activity by Hitachi, Tencor Instruments (U.S. Pat. No. 5,276,498), OSI (U.S. Pat. No. 4,806,774) and IBM (U.S. Pat. No. 5,177,559), there has been no interest in a combination of brightfield and darkfield techniques due to a lack of understanding of the advantages presented by such a technique.
All of the machines that are available that have both brightfield and darkfield capability, use a single light source for both brightfield and darkfield illumination and they do not use both the brightfield and the darkfield images together to determine the defects.
The conventional microscope that has both brightfield and darkfield illumination, has a single light source that provides both illuminations simultaneously, thus making it impossible to separate the brightfield and darkfield results from each other.
In at least one commercially available microscope from Zeiss it is possible to have separate brightfield and darkfield illumination sources simultaneously, however, there is a single detector and thus there is no way to separate the results of the brightfield and darkfield illumination from each other. They simply add together into one combined full-sky illumination.
It would be advantageous to have a brightfield/darkfield dual illumination system where the advantages of both could be maintained resulting in a enhanced inspection process. The present invention provides such a system as will be seen from the discussion below. In the present invention there is an unexpected result when brightfield and darkfield information is separately detected and used in conjunction with each other.